1. Field of the Invention
The present invention relates to a transmission method for a downlink control signal in a MIMO communication system capable of transmitting/receiving data by transmitting different control information regarding a modulation method and an OVSF code to a mobile station from each transmit antenna.
2. Description of the Related Art
A MIMO communication system has been known to perform an operation to transmit/receive data better than a single antenna system, that is, a single antenna vs. a single antenna or a single antenna vs. multiple antennas. In order for the MIMO communication system to perform an improved operation, multiple signals transmitted to multiple receive antennas must not be relevant to one another under an abundant scattering environment. However, if the signals are not completely relevant to one another, the operation ability of the MIMO system is decreased. Hereinafter, one embodiment of a MIMO communication system in accordance with the related art will be explained.
FIG. 1 is a view showing a construction of a vertical bell laboratories layered space time (V-BLAST) system in a MIMO communication system in accordance with the related art.
Referring to FIG. 1, the construction of the vertical bell laboratories layered space time (V-BLAST) system in a MIMO communication system in accordance with the related art will be explained as follows.
The V-BLAST is a kind of MIMO communication system including multiple transmit/receive antennas, in which M antennas are used at a transmitting end and N antennas are used at a receiving end under a condition of M≦N.
At the transmitting end, different signals are transmitted to each antenna so that data sequentially generated can be respectively transmitted from each transmit antenna to each receive antenna. To this end, each transmission data 11 undergoes a vector encoder 10, that is, undergoes a serial-to parallel circuit so as to transmit data sequentially generated to each antenna in parallel. A modulation method of data transmitted to each antenna and the number of channel codes can be differently set. The channel code denotes a code having an orthogonal characteristic like an OVSF code.
In case of using multiple transmit antennas 12, different signals are transmitted to different antennas under a state that an additional signal process is not performed or a space-time code is not used. That is, an additional signal process for enhancing quality to transmit data is not performed at a transmitting end.
That is, different signals are transmitted from each antenna at a transmitting end, and the transmitted signals are received by each receive antenna by using an additional algorithm at a receiving end. The receiving end properly detects the signals transmitted from each antenna. A V-BLAST signal processor 18 of a receiving end shown in FIG. 1 shows a signal processing unit for detecting signals respectively transmitted from M antennas at a receiving end.
An operation of the V-BLAST system will be explained in more detail as follows. The transmitting end of the V-BLAST system transmits different signals with regard to inputted data from each transmit antenna without performing an additional signal process or using a space-time code.
A modulation of each antenna and the number of channel codes such as an OVSF code can be differently set.
That is, if a channel state of a downlink transmitted from a transmitting end antenna is good, data of QAM modulation method and a high coding rate is transmitted by using many OVSF codes. On the other hand, if the channel state is not good, data of QPSK modulation method and a low coding rate is transmitted by using less OVSF codes.
The transmitting end processes each different signal to be transmitted from each different antenna by differently setting each modulation method or the number of OVSF codes, and the receiving end detects each signal differently transmitted from each transmit antenna through a proper signal processing.
The signal processing at the receiving end is performed by calculating a weight vector of a receiving array antenna of a receiving end with regard to each signal transmitted from each transmit antenna by considering a signal transmitted from another transmit antenna as an interference signal at the time of detecting a signal transmitted from a specific transmit antenna, and is performed by removing an influence of a previously detected signal on the receiving end. The signal processing at the receiving end is also performed by detecting signals transmitted from each transmit antenna in the order that a ratio of a signal to interference noise is greater.
As aforementioned, in case that each transmit antenna has a different modulation, coding rate, and the number of OVSFs, the information has to be transmitted to a mobile station through a downlink.
In the conventional high speed downlink packet access (HSDPA) system, the information is transmitted to a mobile station through a HSDPA-shared control channel (HS-SCCH). However, in the HSDPA system, multiple transmit/receive antennas are not constructed differently from the MIMO system, thus transmitting only information for one antenna through the HS-SCCH. On the contrary, in the MIMO system, multiple transmit antennas can have different modulations, coding rates, the number of OVSFs, etc. thereby to require a control signal for each antenna.